Toothed belts generally comprise a body made of elastomeric material, in which a number of longitudinal filiform resistant inserts, also called cords, are embedded, and a number of teeth coated in a coating fabric.
Each belt component contributes to increasing performance in terms of mechanical resistance, in order to reduce the risk of breakage of the belt and increase the specific power that can be transmitted.
The belt coating fabric increases the resistance to abrasion and therefore protects the work surface of the belt from wear due to rubbing between the sides and tops of the belt teeth and the sides and bottoms of the races of the pulley with which the belt interacts.
Furthermore the coating fabric reduces the friction coefficient on the work surface, reduces the deformability of the teeth and above all reinforces the roots of the teeth thus avoiding breakage.
The coating fabric used can consist of a single layer or, alternatively, can be a double layer in order to guarantee greater strength and rigidity. The fabric is normally treated with an adhesive, for example RFL (resorcinol and formaldehyde latex) to increase the adhesion between the body and the fabric.
Currently many drive systems use gears or chains instead of toothed belts. Both gears and chains are, however, complex systems to produce. Both chains and gears are noisier and operate exclusively with oil, furthermore during operation they are subject to greater elongation and therefore replacement with a belt permits greater meshing precision.
In addition to these main disadvantages, both chains and gears are very costly.
For these reasons it is expedient to replace chains and gears with toothed belts without having to perform any other modification to the drive system as a whole; in this case, therefore, the toothed belt would necessarily have to operate in contact with oil or even partially immersed in oil.
Numerous studies have already been carried out on toothed belts to ascertain whether they are able to operate in direct contact with oil. For example the United States patent U.S. Pat. No. 4,099,422 describes a toothed belt designed to be used in an oil bath and comprising a body made of elastomeric material, preferably epichlorohydrin and a double layer of fabric coating the teeth.
The patent application EP0549401 describes a toothed belt comprising a body consisting of a first layer in CSM or ACSM and a second layer on the tooth side consisting of a matrix of HNBR coated with a fabric treated with HNBR.
In contact with oil, in particular during meshing of the belt on the pulley, the elastomeric materials used to form the body of the belt are subject to undesired expansion which causes a reduction in mechanical characteristics, reduced adhesion and, due to dimensional variations, also less efficient meshing. Furthermore none of the known toothed belts are particularly resistant to wear.
All the above problems therefore lead to easier breakage of the belt and therefore a shorter average working life.
No toothed belt in use in contact with oil or partially immersed in oil is therefore able to resist the scheduled duration tests for use in vehicle drive systems.